The present invention relates to ink jet printers and, more particularly, to a printer in which the ink jet print head is shifted parallel to the axis of rotation of a sheet supporting drum during a plurality of rotations of the drum.
Ink jet printers, and the mechanisms by which drops in a plurality of jet drop streams are selectively charged and deflected to produce a print image, are well known and are illustrated in patents such as U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis, and U.S. Pat. No. 3,373,437, issued Mar. 12, 1968, to Sweet et al. Typically, such printers include a print head which generates a plurality of jet drop streams, which streams may be positioned in one or more rows. The print head includes a charge electrode arrangement which selectively induces charges upon the drops in the streams. The charged drops are deflected by an electric field while the uncharged drops pass through the field unaffected. Thus, the charged and uncharged drops are separated into two sets of trajectories. A catcher arrangement is provided in the path of one of the sets of trajectories, with the drops in the other set of trajectories striking a print receiving medium. Printers such as shown in the Mathis patent have utilized a plurality of rows of jet drop streams with the print lines serviced by each of the rows interlacing with those serviced by the others of the rows. This permits high resolution printing to be accomplished on a continuous web of paper or other print receiving material. While such a single pass printing operation provides high printing speed, it may not be particularly advantageous in a printing situation in which extremely high speed printing is not required.
As shown in U.S. Pat. No. 4,112,469, issued Sept. 5, 1978, to Paranjpe et al, the lower printing speed requirements of a copier permit each sheet of copy paper to be printed as the sheet is moved repeatedly past the print head on a rotating drum. In the Paranjpe et al copier, an array of printing nozzles is moved axially along the rotating drum for printing during a plurality of rotations. Simultaneously, an optical scanning system scans an original document and sweeps the image of the document across an array of photosensors arranged to correspond in relative position to the array of printing nozzles. Output signals from the photosensors are supplied to a control unit which controls the printing operation of the nozzles. The nozzles are rather widely spaced, but their spacing is adjusted in accordance with the number of nozzles and with the speed of movement of the print head such that helical print lines, serviced during each rotation of the drum, interlace with previously and subsequently serviced print lines to produce an interlaced, solid printed copy.
Such an arrangement provides significant simplification of the copying apparatus. For example, if a single copy is printed during N rotations of the sheet supporting drum, the number of optical sensors and nozzles required is reduced by a factor of N, as compared with the number of sensors and nozzles which would be required for printing during a single rotation of the drum. Additionally, the control and signal processing circuitry required for the copying apparatus is correspondingly simplified.
In the Paranjpe et al copying device, the sheet supporting drum is rotated by a servo motor controlled by a central control circuit. The print head is mounted on a threaded shaft which is rotated by a second servo motor, which also controls lateral shifting of the optical scanning apparatus. While servo motor control of the print head position with respect to the rotational position of the drum is extremely accurate along the entire path of movement of the print head, small fluctuations in print head position affect significantly the quality of the printed copy. While few such inaccuracies may occur during printing of a copy, if the print lines are not positioned accurately, narrow unprinted strips may be apparent in the printed copy.
U.S. Pat. No. 3,689,693, issued Sept. 5, 1972, to Cahill et al, discloses a plurality of print heads which generate a plurality of jet drop streams positioned in a row, with the streams widely spaced apart and a stepping motor arrangement for shifting the print heads parallel to the axis of rotation of a paper supporting drum. The print heads are intermittently stepped laterally by a relatively small distance after each rotation of the drum such that each jet drop stream prints along a plurality of adjacent print lines on the sheet. Each jet drop stream therefore prints a band of print lines, with adjacent bands being printed by adjacent jet drop streams. The stepping motor arrangment for shifting the print heads is subject to the same position errors discussed above with respect to the Paranjpe et al copier.
Another multiple jet, ink jet printer utilizing printing of a sheet during a plurality of rotations of a drum upon which the sheet is mounted is disclosed in U.S. Pat. No. 4,069,485, issued Jan. 17, 1978, to Martin. The Martin printer generates a plurality of rows of jet drop streams which deposit drops along helical, interlacing print lines. The Martin printer includes a document scanner which alternately scans the document from top to bottom and from bottom to top on successive printing operations. The print head is shifted alternately from left to right and from right to left during successive printing operations. The Martin printer utilizes a servo drive mechanism to control shifting of the print head.
Accordingly, it is seen that there is a need for a simple reliable ink jet printer in which the print head is shifted laterally with respect to a sheet supporting drum, which is capable of printing during movement of the print head in either direction, and in which the print head is accurately positioned during printing.